"Uncorking Pressure" and gas volumes

[BigBilly]

I'm wondering if there is a rule of thumb or a way to guesstimate the max pressures your tube/blast chamber will see based on SAAMI chamber pressures and barrel length that will get you close enough for government work...

There's some interesting information available for M855 at http://www.dtic.mil/ndia/2010armament/W ... pDater.pdf , but I have no idea how or if this could be applied to other rounds or if there are similar studies for other popular chamberings.

It may be the case that it's not necessary to know these things to design a successful silencer and all that's really necessary is to copy subsonic's idea - but I would like to have a greater understanding of what's happening and understand how trustworthy a home-built can is rather than accept empirical evidence from these intertubes that they will remain sound for "thousands of rounds".

Anyone offer any knowledge?

[Historian]

BigBilly, thank you for directing us to this valuable
experiment.

[BigBilly]

BigBilly, thank you for directing us to this valuable
experiment.

I'm sure this had made the rounds at Silencer Talk, I'm just having issues finding more data. Maybe more doesn't exist, but I also thought that if anyone would know it would be someone here.

[CMV]

Interesting physics question. You'd need to know a little more like blast chamber & barrel volume. http://www.ehow.com/how_5149949_calcula ... ssure.html This goes over using Boyle's law.

I think the calculation would give a high number due to the gas exiting the bore since the blast chamber isn't sealed, but it's a starting point maybe & should tell you the max pressure your blast chamber could possibly see. Maybe not - it's been a very long time since I've seen the inside of a classroom...

[Dr.K]

That is a pretty cool experiment.

I have to be honest. I haven't looked up the first pressure figure, or done a pressure vessel calculation for any of the few silencers I've built.

I merely copy the thicknesses that I have been able to find, and not vary too far from that. I'd like to think I'm not alone with this method.

[Tony M.]

If you know some parameters of the load (like powder weight and type used, bullet weight etc) The software program called "quick loads" can estimate pressure extant for given barrel lengths.

This would be particularly useful for someone attempting to create the perfect ammunition for suppressed fire. For instance, I used the program several years ago to develop what has become my small game hunting round in a custom rifle; the rifle is a pistol caliber bolt action made for small game hunting and target practice, I wanted a load that would provide sufficient energy for my needs, but still be hearing safe. Using the software I came up with a load that had less than 1K PSI of pressure behind the bullet as it exited the muzzle. It pushes a 125 grain bullet 975 FPS and sounds quieter than an unsuppressed .22 subsonic round from a 16" barrel. If I wanted to suppress the gun, this would be a perfect load, as the low exit pressure and subsonic velocity would result in easy and effective suppression.

[gunny50]

If you know some parameters of the load (like powder weight and type used, bullet weight etc) The software program called "quick loads" can estimate pressure extant for given barrel lengths.

This would be particularly useful for someone attempting to create the perfect ammunition for suppressed fire. For instance, I used the program several years ago to develop what has become my small game hunting round in a custom rifle; the rifle is a pistol caliber bolt action made for small game hunting and target practice, I wanted a load that would provide sufficient energy for my needs, but still be hearing safe. Using the software I came up with a load that had less than 1K PSI of pressure behind the bullet as it exited the muzzle. It pushes a 125 grain bullet 975 FPS and sounds quieter than an unsuppressed .22 subsonic round from a 16" barrel. If I wanted to suppress the gun, this would be a perfect load, as the low exit pressure and subsonic velocity would result in easy and effective suppression.

Tony,

Quick Loads is used by a lot of people for reloading and load development, I do think that its a great tool to get some first data on gas pressure.

Gunny

[doubloon]

Interesting physics question. You'd need to know a little more like blast chamber & barrel volume. http://www.ehow.com/how_5149949_calcula ... ssure.html This goes over using Boyle's law.
...

Pretty cool. Would it be reasonable to think of the suppressor not as just one ending volume but a series of increasing momentary volumes?

It seems to be generally accepted that projectiles passing through suppressors gain velocity from free bore boost. This is similar to saying the projectile is additionally accelerated by the expanding gas as it momentarily restricts the gas passing through each baffle. So, a suppressor is in effect a series of momentary increasing volumes?

Maybe not enough to matter and the amount of data that would need to be taken into account like the mass of the projectile, clearance around each aperture ... seems easier to just experiment.

[jreinke]

I'm wondering if there is a rule of thumb or a way to guesstimate the max pressures your tube/blast chamber will see based on SAAMI chamber pressures and barrel length that will get you close enough for government work...

There's some interesting information available for M855 at http://www.dtic.mil/ndia/2010armament/W ... pDater.pdf , but I have no idea how or if this could be applied to other rounds or if there are similar studies for other popular chamberings.

It may be the case that it's not necessary to know these things to design a successful silencer and all that's really necessary is to copy subsonic's idea - but I would like to have a greater understanding of what's happening and understand how trustworthy a home-built can is rather than accept empirical evidence from these intertubes that they will remain sound for "thousands of rounds".

Anyone offer any knowledge?

Thanks for posting that link. The photo on page 5 of Doc Dater on the lathe was inspirational! I never would have though to mount a Jacobs collet chuck on the back of the spindle as a cats head. What a great idea!

[whiterussian1974]

Interesting physics question. You'd need to know a little more like blast chamber & barrel volume. http://www.ehow.com/how_5149949_calcula ... ssure.html This goes over using Boyle's law.
...

Pretty cool. Would it be reasonable to think of the suppressor not as just one ending volume but a series of increasing momentary volumes?

It seems to be generally accepted that projectiles passing through suppressors gain velocity from free bore boost. This is similar to saying the projectile is additionally accelerated by the expanding gas as it momentarily restricts the gas passing through each baffle. So, a suppressor is in effect a series of momentary increasing volumes?

Maybe not enough to matter and the amount of data that would need to be taken into account like the mass of the projectile, clearance around each aperture ... seems easier to just experiment.

Perfect explaination of the Physics involved. Mega-bumps!!!

[57fairlane]

I'm wondering if there is a rule of thumb or a way to guesstimate the max pressures your tube/blast chamber will see based on SAAMI chamber pressures and barrel length that will get you close enough for government work...

There's some interesting information available for M855 at http://www.dtic.mil/ndia/2010armament/W ... pDater.pdf , but I have no idea how or if this could be applied to other rounds or if there are similar studies for other popular chamberings.

It may be the case that it's not necessary to know these things to design a successful silencer and all that's really necessary is to copy subsonic's idea - but I would like to have a greater understanding of what's happening and understand how trustworthy a home-built can is rather than accept empirical evidence from these intertubes that they will remain sound for "thousands of rounds".

Anyone offer any knowledge?

Wow, that's a great article.

I may not agree with Dater 100% of the time but he is a scientist no doubt

[BigBilly]

Interesting physics question. You'd need to know a little more like blast chamber & barrel volume. http://www.ehow.com/how_5149949_calcula ... ssure.html This goes over using Boyle's law.

I think the calculation would give a high number due to the gas exiting the bore since the blast chamber isn't sealed, but it's a starting point maybe & should tell you the max pressure your blast chamber could possibly see. Maybe not - it's been a very long time since I've seen the inside of a classroom...

I doubt that Boyle's law is very applicable to the highly dynamic, high pressure and very hot environment in a rifle barrel and I can't figure out how to account for the progressive generation of volume that powder deflagration would cause...

At any rate, I built a spreadsheet based on Boyle's law for 7.62x39. It's pretty basic, assumes a closed system and ignores any gas volume "overage", but it suggests that a 1.652 suppressor with a 3" blast chamber would see a theoretical maximum of about 800 PSI. That seems way too basic to be correct.

Also, entering the values from the Dater experiment into the spreadsheet doesn't work well at all. It seems to be fairly close with the 10" barrel numbers and my guesstimates about the internal volumes of the Gemtech G5, but I think that's just coincidental data crossing...

[MCKNBRD]

I'd have to dig through some notebooks to find it, but I did play around with pressures and trying to calculate what the uncorking pressure would be for a given round in a given barrel length.

I decided that MAX pressure is the worst case scenario, and figured that building a suppressor to stand that would be 'good enough'. If you're using a SAAMI-spec cartridge, SAAMI max pressures are easily obtainable. Use the volume of the cartridge case as an initial volume (where max pressure would be worst), then calculate pressure at a given barrel length by replacing the volume of the chamber with the volume of the barrel's bore.

Exact? Nope. Damn close to Dr. Dater's numbers that he obtained experimentally, though.

Good enough for a Form 1 hobbyist, and that's all I am at this point.

Byrdman

[CMV]

Ok - looking thru the data in the example. @ 15" pressure is roughly 1/7 of chamber pressure and that's only about .6 cu in of volume. The you go into an expansion chamber that's a little more than 10x that volume. Reasonable to assume that would reduce pressure by at least 10x.

So roughly - 55,000psi chamber pressure >>> 7800 psi after expanding into .6 CI >>> 780 psi after expanding into 6.4 CI. 7.62x39 is 10,000 psi less chamber pressure than M855 & there's a larger bore for more expansion. Maybe your 800 psi calculation isn't that far off?

Or maybe I suck at physics & math (quite possible)

Anyway, Pi * Vi = Pf * Vf is the ideal gas law simplified for no change in temp. Assume the rifle chamber & expansion chamber gasses are roughly the same temp.

I have no idea what the 7.62x39 chamber volume is. Pretty small - case capacity is .141 CI so I'll use that. So even though the starting PSI is really big, it's confined to a really tiny space.

45,000psi * .141 = Pf (what you want to know) * 7.55 CI (15" of .308 barrel + 3" of expansion chamber)

I get 840 & that's probably ballpark and close to what you got. I think my 840psi is too high also. Don't know the chamber volume but the bullet is taking up a lot of that volume.

I could be WAAAAYYYY off. But I don't think the pressures in the can are crazy high (although 800+ psi is a lot). A filled aluminum SCUBA tank is around 3,000 psi & look at how thick those are compared to suppressor walls. Look at how thick a gun barrel is at the chamber compared to suppressor walls.

[CMV]

SCUBA tank probably a bad example. Safety factor for probably 2x the pressure and puncture/drop protection so it doesn't grenade, but you still have really thick material to safely contain much less pressure than a gun chamber.

[BigBilly]

SCUBA tank probably a bad example. Safety factor for probably 2x the pressure and puncture/drop protection so it doesn't grenade, but you still have really thick material to safely contain much less pressure than a gun chamber.

Also, the large internal area of an 80 cubic foot SCUBA tank pressured to 3,000 PSI is holding back a far, far larger force than a 0.141 cubic inch x39 casing at 45,000 PSI. You need a lot more brute strength for that sort of work.

On another note, if I build a suppressor from a section of 1.75", 0.049 wall 4130 tubing, use the yield strength rather than the ultimate tensile strength and a safety factor of 4 I get a working pressure of 883 PSI. For my current plan that would be the difference between an 11 oz. tube body and a 15 oz. tube body. This isn't something that I'm going to subject to high volume fire or full auto; this is just a tube that will be used for hunting.

[Grounded]

If you dig around there was an article about testing done by Phil Dater and they mounted a pressure sensor in the blast chamber. and recorded data that way. I want to say it was in the range of 3k psi.

[Grounded]

http://sadefensejournal.com/wp/?p=573

Pressure testing the entrance chamber of a 1.38 inch diameter 5.56mm suppressor on a 10.4 inch barrel. Pressure measurements of 2,998 psi calculated an acceptable safety factor at ambient air temperature and a marginal safety factor after 100 rounds fully automatic fire. The significantly lower chamber pressure on a 14.5 inch barrel (the intended platform for this suppressor) resulted in acceptable safety factors both cold and following fully automatic fire.

[BigBilly]

http://sadefensejournal.com/wp/?p=573

Pressure testing the entrance chamber of a 1.38 inch diameter 5.56mm suppressor on a 10.4 inch barrel. Pressure measurements of 2,998 psi calculated an acceptable safety factor at ambient air temperature and a marginal safety factor after 100 rounds fully automatic fire. The significantly lower chamber pressure on a 14.5 inch barrel (the intended platform for this suppressor) resulted in acceptable safety factors both cold and following fully automatic fire.

My spreadsheet actually models Dater's results with a Gemtech G5 (I'm assuming that's a G5 anyway) on a 10.5" 5.56 upper fairly well, but a 7.62x39 through a 16" barrel equipped with a larger bore home-built silencer would very likely be a significantly lower pressure environment.

[Grounded]

http://sadefensejournal.com/wp/?p=573

Pressure testing the entrance chamber of a 1.38 inch diameter 5.56mm suppressor on a 10.4 inch barrel. Pressure measurements of 2,998 psi calculated an acceptable safety factor at ambient air temperature and a marginal safety factor after 100 rounds fully automatic fire. The significantly lower chamber pressure on a 14.5 inch barrel (the intended platform for this suppressor) resulted in acceptable safety factors both cold and following fully automatic fire.

My spreadsheet actually models Dater's results with a Gemtech G5 (I'm assuming that's a G5 anyway) on a 10.5" 5.56 upper fairly well, but a 7.62x39 through a 16" barrel equipped with a larger bore home-built silencer would very likely be a significantly lower pressure environment.

I think that's a fair assumption

[Hard_ware]

Now that is an accurate way to obtain chamber pressure. Having pressures per chamber(plus volume of chamber), would allow the safety factor to be obtained while cutting volume/weight/type of material to exactly what is needed for the length/diameter that is being targeted. This would vary per barrel length and per load data. After real world testing, coding a modeling program could be constrained accurately if it isn't already. Knowing this info one could use thin aluminium, carbon fiber, stainless etc etc as needed within safety margins. Internal rupture discs could be incorporated to prevent uncalculated failure that could lead to injury. Directing gasses/fragments in a safe direction.

[Grounded]

Now that is an accurate way to obtain chamber pressure. Having pressures per chamber(plus volume of chamber), would allow the safety factor to be obtained while cutting volume/weight/type of material to exactly what is needed for the length/diameter that is being targeted. This would vary per barrel length and per load data. After real world testing, coding a modeling program could be constrained accurately if it isn't already. Knowing this info one could use thin aluminium, carbon fiber, stainless etc etc as needed within safety margins. Internal rupture discs could be incorporated to prevent uncalculated failure that could lead to injury. Directing gasses/fragments in a safe direction.

to simulate that would be crazy. Your talking heated mach flow with particulate.

[Fulmen]

to simulate that would be crazy. Your talking heated mach flow with particulate.

Sure, calculating this from scratch using only input values would be a major challenge. But once you have empirical data things might become easier, most of the parameters would be fixed or vary within a fairly moderate range allowing you to simplify things.

[mollinst]

Jeepers you guys, talk about going to great expense - in time, as well as money.

Just my stand alone Bruel & Kjaer dB metering equipment with mics and calibrators and software programing to run it all through costs over $10K. And that's if you can even find it. Pressure sensors, and all the associated hardware & software to go with them, has to cost at least as much, if not more. Can you do all of the experimentation necessary in a less expensive way?

Bill

[Hard_ware]

to simulate that would be crazy. Your talking heated mach flow with particulate.

Sure, calculating this from scratch using only input values would be a major challenge. But once you have empirical data things might become easier, most of the parameters would be fixed or vary within a fairly moderate range allowing you to simplify things.

Yes. Resolving all possibilities to the molecular level would be a complete waste of time IMHO, I could care less what a mouse fart in the field would do the the gasses escaping the end of the suppressor " Just the facts, stick to the facts " Only variable would be chamber size and Load data, even the thermal transfer to the surrounding surfaces is not needed. Just a pressure from empirical data vs chamber size with bore size, and a correlation to the gas volumes produced by the different loads. This would allow some good prototyping not just destructive testing. Just a thought. Back to my cheap a$$ freeze plugs and tail pipe